AMD Ryzen 7000 Series performs better with Spectre V2 Mitigations enabled

Re:AMD Ryzen 7000 Series performs better with Spectre V2 Mit... by stevefan1999 on Saturday October 08, 2022 @02:13AM (#33130208)

meh, maybe they just flush TLB every time in hardware level if you disabled the mitigations, ane disabled the hardware flush if the software side can handle it

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Re:AMD Ryzen 7000 Series performs better with Spectre V2 Mit... by bmacho on Saturday October 08, 2022 @07:23AM (#33131415)

So mitigations = off when they do flush, and mitigations = on when they use previous data and weird shenanigans?

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Re:AMD Ryzen 7000 Series performs better with Spectre V2 Mit... by Cloudef on Saturday October 08, 2022 @08:38AM (#33131799)

Yeah it seems like hw migtation is slower than software

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Re:AMD Ryzen 7000 Series performs better with Spectre V2 Mit... by staticassertion on Saturday October 08, 2022 @03:29PM (#33135246)

This is basically my guess. In the event of no mitigations you have to pessimistically flush TLB. In the case of mitigations the flush only has to happen in specific instances, which PCID significantly helps with but requires cooperation from both hardware and software.

So maybe even something like "if PCID is not set, flush the cache" vs "if PCID changes, flush the cache".

I think that would certainly account for this, and they could have improved the performance with PCID by increasing the PCID cache size.

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